A radio communication system such as LTE (Long Term Evolution), which is currently undergoing standardization in 3GPP (Third Generation Partnership Project), requires not only an S1 connection that is a logical transmission path between a radio base station (hereinafter referred to as a “LTE base station”) and a core network, but also an X2 connection that is a logical transmission path between radio base stations in order to achieve a quick handover, reduction in a processing load on the core network, and so forth.
In order to establish X2 connections, an LTE base station acquires information on other neighboring LTE base stations and retains a neighboring base station list (hereinafter referred to as a “neighbor list”) as a list of the information on the other LTE base stations. The other neighboring LTE base stations are other LTE base stations located at short distances from the LTE base station. Only after retaining the neighbor list, the LTE base station can control establishment of the X2 connections, load balancing with the LTE base stations, restrictions during a maintenance work, and the like.
An ANR (Automatic Neighbor Relation Function) is one of conceivable methods for the LTE base station to acquire the neighbor list. The ANR is the method in which the LTE base station receives a measurement report transmitted from a radio terminal either periodically or on an event basis and acquires a neighbor list based on the measurement report (see Non-patent Document 1, for example).
Moreover, the LTE base station can request the radio terminal to perform a handover based on the measurement report from the radio terminal. The measurement report contains an ID of an LTE base station which is a sender of a signal (a BCH signal) received by the radio terminal through a broadcast channel (BCH), information on radio field strength of the signal, and the like. Based on the ID of the LTE base information and the information on the radio field strength contained in the measurement report, the LTE base station can identify an LTE base station for a handover destination and instruct the radio terminal to perform a handover at an appropriate timing.
In addition, the LTE radio communication system has a data forwarding function for performing handovers with few packet losses. The data forwarding function is a function with which just before a handover, an LTE base station forwards data, which is yet to be transmitted from the LTE base station to a radio terminal, to another LTE base station of a handover destination via an X2 connection. The LTE base station of the handover destination transmits the data, which is received via the X2 connection, through a radio channel to the radio terminal that is newly connected as a consequence of the handover. Thus it is possible to prevent packet losses in the handover.
For the LTE radio communication system, there are prescribed above-described handover involving data forwarding via an X2 connection (hereinafter referred to as an “X2 handover”) and a handover involving data forwarding via an S1 connection (hereinafter referred to as an “S1 handover”).